Was Gol D Roger as strong as people said he was, or was he just loved by fate like Luffy is?
Well, there are a few pieces of evidence that indicate that Roger was as strong as 'people' say. I'll just list them out.
It was mentioned by Roger, without contradiction, that both him and Garp have nearly killed each other several times. We know that in Garp's younger days, he was at least considered to be a viable candidate to become an Admiral. This was shown during the Battle of Edd War, when the Fleet Admiral Kong offered Garp a promotion and Garp declined that. So, we can at least say that Roger was very capable of going up against an Admiral at this time.
About 38 years ago (or was it 40?), Roger and Garp teamed up to take down the Rocks Pirates. Granted, we don't know what exactly this entails but it's very likely that the both of them (along with the rest of the Roger Pirates probably) had to go up against Big Mom, Kaido, Shiki, Whitebeard, Wang Zhi and Rocks himself. Considering the fact that they ended up winning the battle and Rocks apparently met his end there, it's safe to say that Roger would've surely been considered one of the strongest pirates at that point.
In Oden's flashback, it was shown that both Roger and Whitebeard were at a point where they were able to use Advanced forms of Haki at will and seemingly without any restraints. It seemed to be the case that they could do so without having to exert much effort into it as well. Mastery of such advanced levels of Haki surely counts as a good show of strength.
Finally, we know that Rayleigh, as an old man, had the power to go toe-to-toe against an Admiral. We don't know if he would have won that fight against Kizaru (there are way too many variables that are missing, though I'm of the opinion that it would've ended in a stalemate). But it's very clear that Rayleigh, in his prime, would've been easily able to fight against an Admiral without too many issues. If Roger was not significantly stronger than Rayleigh, then he would've at least been strong enough for Rayleigh to still refer to him as his Captain.
Basically, all of this counts as pretty solid evidence that Roger was as strong as people said. It is interesting to note that in the current context of One Piece, people actually use Whitebeard as a measure of power. That is, they typically measure the strength of a given pirate based on how strong Whitebeard used to be in his prime.
If he is not, he is stronger. Here are some reasons why.
You remember how it was said Whitebeard was as strong as Roger. That was because of Whitebeard's devil fruit. The same devil fruit that had marineford turning into a past version of the Dressrosa arc. That was when whitebeard was sick and old and was dying slowly. That means that it was stronger when he was younger and healthy and roger was sick when whitebeard was young and healthy but they had matched strength. Whitebeard would have been able to beat marineford and the shichibukai alike with the help of Marco and Jozu alone if he was in his prime. Plus Roger had no devil fruit.
You remember Big Mom and Kaido. When they clashed in Wano. (I think or was it in Whole Cake island. I don't remember Kaido going to Whole Cake island though but I think Big Mom chased Luffy all the way to Wano.) It split the skies, right. Well Roger and Whitebeard split the sky with a clash as well, but it didn't even touch each other. I think it was due to a bit of Conqueror's haki but I don't know for sure.
He has Armament, Observation, and Conqueror's haki. You can either have Conqueror's haki at birth or become strong enough to use it. I believe that he obtained through true training and not birth.
He has defeated Rocks D Xebec. Though he did need help, he did around I think 20%-80% of the defeat.
He is above Yonko level.
He start a whole new era with a few words. Not everyday pirates can do that.
He turned himself in. The navy said they captured him so they can look strong but he actually turned himself in.